Timing Channel Circuitry for Creating Pulses in an Implantable Stimulator Device

ABSTRACT

Timing channel circuitry for controlling stimulation circuitry in an implantable stimulator is disclosed. The timing channel circuitry comprises a addressable memory. Data for the various phases of a desired pulse are stored in the memory using different numbers of words, including a command indicative of the number of words in the phase, a next address for the next phase stored in the memory, and a pulse width or duration of the current phase, control data for the stimulation circuitry, pulse amplitude, and electrode data. The command data is used to address through the words in the current phase via the address bus, which words are sent to a control register for the stimulation circuitry. After the duration of the pulse width for the current phase has passed, the stored next address is used to access the data for the next phase stored in the memory.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation application of U.S. patent application Ser. No.14/858,554, filed Sep. 18, 2015 (U.S. Pat. No. 9,468,771), which is acontinuation application of U.S. patent application Ser. No. 14/636,380,filed Mar. 3, 2015 (U.S. Pat. No. 9,144,687), which is a divisionalapplication of U.S. patent application Ser. No. 13/847,676, filed Mar.20, 2013 (U.S. Pat. No. 9,008,790), which was a non-provisionalapplication claiming priority to U.S. Provisional Patent ApplicationSer. Nos. 61/654,603, filed Jun. 1, 2012, and 61/639,814, filed Apr. 27,2012. Priority is claimed to these applications, and they are allincorporated herein by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates generally to implantable medical devices,and more particularly to improved timing channel circuitry for creatingpulses in an implantable stimulator device.

BACKGROUND

Implantable stimulation devices generate and deliver electrical stimulito nerves and tissues for the therapy of various biological disorders,such as pacemakers to treat cardiac arrhythmia, defibrillators to treatcardiac fibrillation, cochlear stimulators to treat deafness, retinalstimulators to treat blindness, muscle stimulators to producecoordinated limb movement, spinal cord stimulators to treat chronicpain, cortical and deep brain stimulators to treat motor andpsychological disorders, and other neural stimulators to treat urinaryincontinence, sleep apnea, shoulder sublaxation, etc. The descriptionthat follows will generally focus on the use of the invention within aSpinal Cord Stimulation (SCS) system, such as that disclosed in U.S.Pat. No. 6,516,227. However, the present invention may findapplicability in any implantable stimulator.

As shown in FIGS. 1A and 1B, a SCS system typically includes anImplantable Pulse Generator (IPG) 100, which includes a biocompatibledevice case 30 formed of a conductive material such as titanium forexample. The case 30 typically holds the circuitry and a battery forpowering the IPG, although IPGs can also be powered via external RFenergy and without a battery. The IPG 100 includes at least oneelectrode array 102 containing several electrodes 106. The electrodes106 are carried on a flexible body 108, which also houses the individualelectrode leads 112 coupled to each electrode. In the illustratedembodiment, there are eight electrodes on array 102, labeled E₁-E₈,although the number of arrays and electrodes is application specific andtherefore can vary. The array 102 couples to the IPG 100 using a leadconnector 38, which is fixed in a non-conductive header material 36, andwhich can comprise an epoxy for example. A communication coil 13 allowsfor telemetry of data to and from the IPG 100 from a device external tothe patient, as is well known. An additional coil may be present toallow the battery to be recharged from an external device, as is alsowell known.

FIGS. 2A-2C illustrate typical pulses used in an IPG such as IPG 100,and the circuitry used to construct the pulses. Shown in FIG. 2A is abi-phasic current pulse that issues a constant current of a givenamplitude (amp) and pulse width (pw) between electrodes E1 and E2. Eachpulse comprises a plurality of pulse phases, and is repeated in a periodat a frequency f. The amplitude, pulse width, and frequency of thepulses can be patient-dependent, and may be arrived at byexperimentation—i.e., based on what provides good therapy for thepatient.

Circuitry for creating the pulses is shown in FIG. 2B and comprisestiming channel circuitry 150 and stimulation circuitry 160 coupled via adigital bus 155. Stimulation circuitry 160 comprises a current source(PDAC) 83, a current sink (NDAC) 84, a switching matrix 85, and passiverecovery switches 86, each of which will be explained subsequently. ThePDAC 83 and NDAC 83 are so named because the amplitude of the analogcurrent they source or sink is digitally controllable (hence, they areDigital-to-Analog Converters, or DACs), and because they are typicallymade from P-channel and N-channel transistors respectively.

Therapeutically meaningful phases of the example pulses in FIG. 2A willbe explained first, starting with phase 1. In phase 1, electrode E1 actsas the cathode or sink for the current pulse, while electrode E2 acts ofthe anode or source of the current pulse. Thus, sourced current of thedesired amplitude is issued from the PDAC 83 to E2 by closing anappropriate switch in the switching matrix 85, while sunk current ofthat same amplitude is drawn into the NDAC 84 from E1 by closing anappropriate switch in the switching matrix 85. This causes the currentto flow from E2 to E1 through the patient's tissue (R). Notice that thepulses at E1 and E2 during phase 1 have the same amplitude (although ofopposite polarities) and the same pulse width (pw), so that an excess ofcharge does not build up in the patient's tissue, R.

Because the pulse is bi-phasic, phase 1 is eventually followed by asecond phase (phase 2), which in this example comprises a reversal ofthe polarity of the pulses at the two electrodes. Thus, in phase 2, E1now acts as the anode, and E2 as the cathode, such that current nowflows through the tissue R in the opposite direction. Phase 2, as isknown, is useful to recover any excess charge that might have formed onany capacitances in the current path, such as the decoupling capacitorsC1 and C2 coupled to electrodes E1 and E2 respectively (FIG. 2B).Connecting decoupling capacitors C1-CN to the electrodes E1-EN acts as asafety measure to prevent direct DC current injection into the patient.However, the storage of charge across such capacitors during theprovision of the current is generally undesired. Reversing the currentthrough those capacitances during phase 2 seeks to actively recover suchstored charge, and thus phase 2 is sometimes referred to as an “activerecovery” phase. To actively recover stored change in this fashion, itis preferable that the same amount of charge be passed in phase 2 as waspassed in phase 1, which is most easily (and symmetrically) done bymaking the amplitude and pulse width of phase 2 equal to phase 1.However, one skilled will realize that symmetry between the pulses inphase 1 and phase 2 is not strictly necessary. For example, a pulse oflower amplitude and longer pulse width in phase 2 can also equate to thesame amount of charge passed during phase 1. The polarity reversal ofthe pulses in phase 2 can be accomplished by the switching matrix 85,which in phase 2 will couple E1 to the PDAC 83, and E2 to the NDAC 84.

To ensure complete recovery of any stored charge, the active recoveryphase (phase 2) can be followed by a passive recovery phase, as shown inFIG. 2A. In this passive recovery phase, the decoupling capacitors C1-C2connected to previously-active electrodes E1 and E2 are shorted to acommon potential via passive recovery switches 86 (FIG. 2B). In theexample illustrated, this common potential, Vbat, comprises the voltageof the battery within the IPG 100, although other reference potentialscould be used as well. Shorting the capacitors to Vbat effectivelyshorts them through the patient's tissue, and thus equilibrates anystored charge to assist in charge recovery. Some architectures may shortonly the previously-active electrodes by closing only the passiverecovery switches 86 coupled to those electrodes, while otherarchitectures will short all of the electrodes by closing all of thepassive recovery switches 86.

Other pulse phases in each period are shown in FIG. 2A. Preceding pulse1 is a pre-pulse phase, which is of low amplitude and long duration, andof opposite polarity to the phase 1 pulse that follows it.Experimentation suggests that the use of such a pre-pulse can help toassist in recruiting deeper nerves in an SCS application, although useof such a pre-pulse is not strictly necessary. An interpulse periodbetween phases 1 and 2 of short duration allows the nerves to stabilizeafter being stimulated in phase 1. A quite phase follows the passiverecovery phase, and essentially acts as a waiting phase before the nextperiod of pulses issues. The duration of the quite phase will depend onthe durations of the phases that precede it in the period, as well asthe frequency (f) at which the periods issue. There may be other pulsesphases for different purposes; the phases shown in FIG. 2A are merelytypical.

Referring again to FIG. 2B, the various phases of each period arecontrolled by timing channel circuitry 150, which digitally controls thevarious elements in the stimulation circuitry 160 via digital bus 155.There can be several timing channels circuitries 150 operating in agiven IPG 100, defining other pulse sequences perhaps issuedconcurrently on other electrodes. However, only one timing channelcircuitry 150 is illustrated for simplicity. The timing channelcircuitry 150 receives and stores the data necessary to define thevarious phases in each pulse period. Such pulse parameters are providedto the timing channel circuitry 150 from control circuitry (e.g., amicrocontroller) 110 via a digital bus 115. The control circuitry 110 inturn typically receives the pulse parameters wirelessly from an externaldevice, such as an external controller. An external controller istypically a hand-held device usable by a patient or her clinician toselect the pulse parameters, such as amplitude, pulse width, frequency,the electrodes to be used, and whether they are to act as anodes orcathodes.

FIG. 2C shows further details of the timing channel circuitry 150. Shownare a timer 170 and a register bank 180. The timer 170 stores thedurations (pulse widths) of the phases in the period, while the registerbank 180 stores control information, amplitude, active electrode, andelectrode polarity information for the phases. Thus, a first register inthe timer 170 stores the pulse width of the first pulse phase in theperiod (the pre-pulse (pw_(pp)) in the example of FIG. 2A), and thecorresponding first register in the register bank 180 stores its controlinformation (cntl_(pp)) amplitude (amp_(pp)), active electrode, andelectrode polarities. A second register in the timer 170 stores thepulse width of the next pulse phase (phase 1 (pw_(p1))), and thecorresponding second register in the register bank 180 stores thecontrol information (cntl_(p1)), amplitude (amp_(p1)), active electrode,and electrode polarity for phase 1. Data for subsequent pulse phases(interphase (ip), phase 2 (p2), passive recovery (pr), and quite (q))are similarly stored in the timer 170 and register bank 180. The timer170 may comprise a state machine in one example.

The control data in the registers (cntl) contains information necessaryfor proper control of the stimulation circuitry 160 for each phase. Forexample, during the passive recovery phase, the control data (cntl_(pr))would instruct certain passive recovery switches 86 to close, and woulddisable the PDAC 83 and the NDAC 84. By contrast, during active phases,the control data would instruct the passive recovery switches 86 toopen, and would enable the PDAC 83 and the NDAC 84.

Each register in the register bank 180 is, in one example, 96 bits inlength, with the control data for the phase in the first 16 bits, theamplitude of the phase specified in the next 16 bits, followed by eightbits for each electrode. Each of the eight electrode bits in turnspecifies the polarity (P) of the electrode in a single bit, with theremaining 7 bits specifying the percentage (%) of the amplitude thatthat electrode will receive. Thus, for the pre-pulse phase, the polaritybit P for E1 would be a ‘1’, specifying that that electrode is to act asan anode, and thus will receive sourced current of the specifiedamplitude (amp_(pp)) from PDAC 83. The remaining seven bits for E1 woulddigitally represent 100%, indicating that E1 is to receive the entiretyof the sourced current during the pre-pulse phase. (In more complicatedexamples, the sourced or sunk currents could be shared betweenelectrodes, and thus smaller percentages would be indicated in thetrailing seven bits for those electrodes). The polarity bit P for E2during the pre-pulse phase would be a ‘0’, specifying that thatelectrode is to act as a cathode, and thus will sink current ascontrolled by NDAC 84. Again, the remaining seven bits for E2 woulddigitally represent 100%, indicating that E2 is to receive the entiretyof the sunk current during the pre-pulse phase.

The other registers in register bank 180 are programmed similarly foreach phase. For example, all of the bits for E3-E8 in all of theregisters would be set to zero for the example pulse phases of FIG. 2A,because those electrodes are not implicated. The amplitudes for theinterphase (amp_(ip)), passive recovery (amp_(pr)), and quite (amp_(q))phases would be set to zero as those phases do not require the PDAC 83or NDAC 84 to actively issue any current.

The goal of the timing channel circuitry 150 is to send data from anappropriate register in the register bank 180 to the stimulationcircuitry 160 at an appropriate point in time, and this occurs bycontrol of the timer 170. As noted earlier, the pulse widths of thevarious phases are stored in the timer 170. Also stored at the timer isthe frequency, f, the inverse of which (1/f) comprises the duration ofeach period. Knowing this period, the timer 170 can cycle through thedurations of each of the pulse widths, and send the data in the registerbank 180 to the stimulation circuitry 160 at the appropriate time. Thus,at the start of the period, the timer 170 enables a multiplexer 190 topass the values stored in the first register for the pre-pulse to bus155, which enables stimulation circuitry 160 to establish the pre-pulsesat electrodes E1 and E2. After time pw_(pp) has passed, the timer 170now enables the multiplexer 190 to pass the values stored in the secondregister for phase 1 to the stimulation circuitry 160 to establish thephase 1 pulses at the electrodes. The other registers are similarlycontrolled by the timer 170 to send their data at appropriate times.This process of cycling through the various pulse phases continues, andeventually at the end of quite phase, i.e., at the end of pw_(q), thetimer 170 once again enables the pre-pulse data, and a new period ofpulses is established.

This approach for controlling the stimulation circuitry 160 inaccordance with each phase of the pulse period is adequate, but theinventors have found that this approach also suffers from certainshortcomings. A significant shortcoming is the lack of flexibility thatthe timing channel circuitry 150 provides to define more complex pulses.Take the pulse illustrated in FIG. 3 for example. This pulse generallycontains the same phases as illustrated in FIG. 2A, but the currentsspecified during the therapeutically-meaningful phases (phases 1 and 2)are not constant currents, but instead comprise ramped currents.Experimentation suggests that such ramped pulses can be therapeuticallyuseful in certain situations. Yet, the timing channel circuitry 150 asillustrated in FIG. 2C cannot direct the stimulation circuitry 160 toproduce ramped pulses of this type, because only a constant amplitudecan be specified for each phase in the register bank 180.

The inventors recognize that one could approximate a ramped pulse usingthe architecture of FIG. 2C by breaking the ramped pulse phases into aplurality of shorter phases, each with a constant current defining theramp in a stair-wise fashion, as shown superimposed on phases 1 and 2for electrode E1 in FIG. 3. However, this would require expanding theregister bank 180 to accompany the increased number of phases in theperiod. Assume for example that to form a suitably-smooth ramp it wouldbe necessary to parse both of phases 1 and 2 into ten smaller phases.The period would then comprise 24 different phases: the 20 phases neededin each of phases 1 and 2, the pre-pulse phase, the inter-pulse phase,the passive recovery phase, and the quiet phase. Because the registerbank 180 must contain a register for each phase in the period, that bank180 would then need 24 different registers. The 96 bits needed for eachregister in the bank 180 typically comprise flip flops, and so in thisexample 2304 (96*24) flip flops would be required, or more if the IPG100 supports further numbers of electrodes. If the design of theintegrated circuit is already fixed, and contains less than 24registers, then the stair-wise approximation of the ramped pulsesposited above simply could not be realized in that design. In short, theintegrated circuit designer must either provide an undue number ofarea-intensive registers in register bank 180 to potentially handle thedesign of complex pulses in the future, or provide a limited number ofsuch registers and forego the use of such complex pulses; neither optionis desirable.

Furthermore, flip flops require significant layout area on theintegrated circuitry in which the stimulation circuitry 160 is typicallyformed, which is undesirable. Further, the flip flops consume power whenthey are clocked, which can lead to complexity in gating the clocks tosave power. The problem of excessive layout space is compounded by thefact that there may be more than one timing channel circuitry 150operating in the device, as mentioned earlier. If the design of theintegrated circuit is already fixed, and contains less than 24registers, then the stair-wise approximation of the ramped pulsesposited above simply could not be realized in that design. In short, theintegrated circuit designer must either provide an undue number ofarea-intensive registers in register bank 180 to potentially handle thedesign of complex pulses in the future, or provide a limited number ofsuch registers and forego the use of such complex pulses; neither optionis desirable.

The multiplexer 190 also takes considerable layout and control. Themultiplexer 190 must generally receive all 96 bits from each register,and so there are a total of 576 (96*6) bits being input to themultiplexer 190 in the example of FIG. 2C. The multiplexer 190 must inturn drive all of the 96 selected bits to the stimulation circuitry 160,which places power constraints on the system. Again, if there is morethan one timing channel circuitry 150, such complexities and powerissues are multiplied.

Also regrettable in the inventor's opinion is the fact that, dependingon the particular phases in the pulse, certain of the register values inthe register bank 180 will necessarily be irrelevant. For example, theamplitude and electrode values will necessarily be zero (or don't carevalues) for the illustrated interpulse, passive recovery, and quitephases. If the functions of the various phases are known in advance, thenumber of flip flops needed in the register bank (and thus the totalnumber of inputs to the multiplexer 190) could be reduced. For example,if it is known in advance that the third register will always hold datafor the interphase, the fifth register will always hold data for passiverecovery, and the sixth register will always hold data for the quitephase, then those registers need not contain flip flops for holding theamplitude and electrode values. But reducing the number of flip flops ina given register limits future flexibility of the device. For example,if the registers just discussed above were made smaller, then thoseregisters could not be used in the future for active stimulation phases.

A better solution is therefore needed to the aforementioned problems,and is provided by this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B show an implantable pulse generator (IPG), and an electrodearray coupled to the IPG in accordance with the prior art.

FIGS. 2A-2C illustrate example pulses created by the prior art IPG 100of FIG. 1, and the timing channel circuitry used to send pulseparameters to stimulation circuitry.

FIG. 3 illustrates a ramped pulse which is not easily handled by thetiming channel circuitry of FIGS. 2B and 2C.

FIG. 4 illustrates improved timing channel circuitry employing a memoryfor holding the pulse parameters.

FIG. 5 illustrates how data is populated in the memory to form thepulses of FIG. 2A.

FIG. 6 illustrates how data is populated in the memory to form the ramppulses of FIG. 3.

FIG. 7 illustrates the extension of the improved timing channelcircuitry to multiple timing channels.

DETAILED DESCRIPTION

Improved timing channel circuitry for controlling stimulation circuitryin an implantable stimulator is disclosed. The timing channel circuitrycomprises a memory addressable by an address bus. Data for the variousphases of a desired pulse are stored in the memory using differentnumbers of words, including, in one example, at least a commandindicative of the number of words in the phase, a next address for thenext phase stored in the memory, and a pulse width or duration of thecurrent phase. Additional words stored in the memory for the currentphase may include control data for the stimulation circuitry, pulseamplitude, and electrode data. The command data is used to addressthrough the words in the current phase via the address bus, which wordsare sent by a data bus to a control register for the stimulationcircuitry, which, depending on the data, may form a pulse phase at theelectrodes. After the duration of the pulse width for the current phasehas passed, the stored next address is output to the address bus toaccess the data for the next phase stored in the memory, and the processrepeats to form a next pulse phase. The memory is physically smaller perbit than registers typically used to store data for each of the pulsephases, and therefore a larger number of pulse phases can beaccommodated, which allows for the production of more complicatedpulses. The data stored for each phase generally doesn't includeunnecessary data, such as electrode data for phases not requiringstimulation, which further economizes storage of the data for the phasesin the memory. Because data is sequentially sent one word at a time tothe control register via the data bus, the complexity of multiplexingand control are reduced.

The new timing channel circuitry 200 for an IPG 100 is shown in a firstexample in FIG. 4. The various pulse parameters are stored in adual-port memory 202, which may comprise a random-access memory forexample. As one skilled in the art will understand, a dual-port memorycan interface with two data and address buses which can runconcurrently. As shown, bus 115 comprises a first address/data bus, andthe pulse parameters are written from this bus 115 into the memory 202from control circuitry 110. The second address/data bus is shownseparately as a data bus 204 and an address bus 224 to better illustrateoperation of the circuitry. The basic goal of the data bus 204 andaddress bus 224 is to read the stored pulse parameters from the memory202 and to send them to stimulation circuitry 160 at the correct time toform the desired pulse phases.

Data for each of the pulse phases is stored in words in the memory 202at different addresses, and each of the phases can comprise differentnumbers of words depending on the function of that phase. In theillustrated example, each word is 32-bits, and in example phase 4, fouraddresses (addr(x)-addr(x+3)) are shown holding four words of data(word1-word4) for that phase. When addressed, the data for each of thesewords is passed to one of registers 208 or 210 via the data bus 204.Specifically, data for the first word (word1) is passed to register 210,while data for the subsequent words (word2-word4) are passed toregisters 206.

Each of the words has a particular pre-defined data structure and orderin which they are stored in the memory 202 for each phase. The firstword in each phase (word1) includes 8 bits defining a command (cmd), 8bits defining an address in the memory 202 for a next phase (“nextaddress” or naddr, described further below), and the pulse width (pw)for the phase. The next word in each phase (word2) includes 16 bitsdefining control for the stimulation circuitry 160 (cntl), and 16 bitsdefining the amplitude of the pulse for the phase (amp). The next wordscomprise electrode and polarity data of the pulse for the phase. Thus,word3 contains 8 bits for each of electrodes E1-E4, with each includinga polarity bit and seven trailing bits defining the percentage of theamplitude that electrode receives, as discussed earlier in conjunctionwith the prior art. Word4 likewise contains similar data for electrodesE5-E8.

The data for each phase is written to the registers 206 and 210 by astimulation controller 214, which operates as follows. When the firstword in each phase is addressed (e.g., addr(x)), register 210 ispopulated with the command (cmd), the next address (naddr), and thepulse width (pw). The next address, naddr, is stored in next addressregister 220, which comprises the address in the memory 202 for the nextpulse phase. For example, and assuming that the data for phase 4 iscurrent being read, naddr would comprise the address in the memory 202for the first word of phase 5. The command data (cmd) is sent to thestimulation controller 214. As will be explained in further detail withrespect to FIG. 5, the command indicates the number of words stored forthe phase, which can vary. The stimulation controller 214 determinesthis number of words (216) to control a multiplexer (mux) 218 to addressthe memory 202 via the address bus 224. For example, cmd in the exampleof FIG. 4 would inform the stimulation controller 214 that the currentphase consists of three more words (word2-word4).

The current address (addr(x)) is incremented (222) in accordance with asystem clock (Clk), which is also received by the stimulation controller214. The mux 218 passes this incremented address to the address bus 224as many times as necessary to address all of the remaining words in thephase. Thus, mux 218 in this example is controlled to passaddr(x+1)-addr(x+3) to the memory 202, which results in word2-word4being sequentially loaded into registers 206 in subsequent cycles of thesystem clock, Clk. Such data, once latched, can be passed in unison to acontrol register 208 for the stimulation circuitry 160. Registers 206and 208 can also be collapsed into a signal circuit. Regardless, onceall words in the phase have been addressed and loaded, the stimulationcontroller 214 can enable (en) the control register 208 to issue theappropriate data via bus 155 to the stimulation circuitry 160 to formthe appropriate pulse phase (phase 4) at the electrodes. Notice that thedata sent to the stimulation circuitry 160 via bus 155 is the same(cntl, amp, E1-E8) as was used in the prior art circuitry (FIG. 2C), andas a result, stimulation circuitry 160 can remain unchanged.

Once the pulse phase is established at the stimulation circuitry 160,the stimulation controller 214 waits to take further action until theduration of that phase has elapsed. This occurs in the illustratedexample by use of a pulse width counter 212, which receives the pulsewidth (pw) of the current phase from register 210. As the period of thesystem clock Clk is known, the counter 212 can know when the duration ofthe pulse width has expired, at which time it issues an end of pulsewidth command to the stimulation controller 214. When this occurs, thestimulation controller 214 now controls the mux 218 to pass to theaddress bus 224 the next address (naddr) that had been stored inregister 210 previously. As discussed earlier, that next addresscomprises the address of the first word in the next phase (phase 5).

When the memory 202 receives this first address of the next phase (phase5), the process described above repeats: a new word1 for phase 5 isloaded into the register 210, including a next address (for phase 6), anew command (which informs concerning the number of words stored forphase 5), and a new pulse width (for phase 5). Mux 218 under control ofthe stimulation controller 214 provides addresses for the remainingwords in phase 5 as assisted by the address incrementer 222. These newaddresses populate registers 206 with the appropriate data for phase 5,which data is then passed to stimulation circuitry 160 to establish thephase 5 pulse, etc.

The system clock, Clk, is typically on the order of a Megahertz or more,and so the period of the system clock (less than a microsecond) istypically small compared to the durations of the pulse widths of thevarious pulse phases (on the order of tens of microseconds or more). Asa result, the timing channel circuitry 200 essentially worksinstantaneously to provide the pulse parameters to the stimulationcircuitry 160 from the memory 202.

FIG. 5 shows further details of the memory 202 and how it can bepopulated to form the example pulses shown in FIG. 2A. As discussedearlier, the pulses in FIG. 2A contain six phases, and the data for eachof the phases are stored as shown. Active phases requiring the PDAC 83and NDAC 84 to generate pulses (such as pre-pulse, phase 1, and phase 2)are denoted as such by command cmd1 in their first words. This commandinforms the stimulation controller 214 that there are three additionalwords associated with these phases, so that addressing can be controlledas discussed above. These additional words, as noted earlier, includecontrol data, amplitudes, and electrode polarities and percentages.

Notice that the next address stored in the first word of each phasepoints to the first word in the next phase, which allows the stimulationcontroller 214 to addresses such next phase as described above. Thus,the next address stored for the pre-pulse phase (in addr(1)) points tothe first address in phase 1 (addr(5)). The next address stored forphase 1 (in addr(5)) points to the first address for the inter-phase(addr(9)), and so on.

Phases requiring only delay but not active generation of currents, suchas the inter-phase and the quiet phase, are denoted by command cmd0 intheir first words. Because such phases do not impact stimulationcircuitry 160, they may be stored in memory 202 and interpreted by thestimulation controller 214 as only a single word, including theirdurations, and the next address.

The passive recovery phase is denoted by command cmd2 in its first word,which requires two words in the illustrated example. This phase does notrequire electrode data (as found in word3 and word4 of the activephases), but does require control data (cntl_(pr)) to inform thestimulation circuitry 160 to close certain of the passive recoveryswitches 86 (FIG. 2B). Because passive recovery is not associated withan amplitude, that field in word2 can be populated with don't-carevalues, or simply set to zero.

Other commands (cmd) are possible, but are not illustrated. For example,other commands may instruct stimulation circuitry 160 to measure thevoltages at the electrodes either during stimulation or during periodsof no stimulation. Such measurements can be useful for taking impedancemeasurements (e.g., for checking the continuity of the electrodes), forsetting the power supply voltage (V+) for the DACs 83 and 84 (FIG. 2B),or for monitoring electrical activity in the tissue of the patient (suchas seizures in a deep brain stimulation application). See, e.g., U.S.Pat. Nos. 7,444,190 and 6,993,384. Other commands may cause stimulationcircuitry 160 to burst the currents at the electrodes, i.e., byfractionalizing the pulses into higher-frequency micropulses. See, e.g.,U.S. Pat. No. 7,890,182. Such other commands may require differentnumbers of words in memory 202, and different control data (cntl),depending on what aspects of stimulation circuitry 160 require control.

With an example of the improved timing channel circuitry 200 understood,advantages over the prior art discussed in the Background can beappreciated. First, the memory cells in memory 202 are generally smallerthan the flip flops used in the register bank 180 of the prior art(180), and thus a greater number of memory locations can be provided tohold and define the pulse parameters. As such, memory 202 providesgreater flexibility to produce pulses with more-complex shapes. Take theramping pulse of FIG. 3 for instance, which as noted earlier was notfeasible to produce in the prior art without an undue number ofregisters in register bank 180. Such ramped pulses can be more easilystored in memory 202, as shown in FIG. 6, which shows the data for aramped phase 1 pulse. The ramped pulse increases from 3 mA to 7.5 mAover a duration of 10 ms, and so 10 smaller phases of 1 ms (phases 1-1to 1-10) increment the amplitude by 0.5 mA each time. The addedflexibility provided by the increased number of memory locations inmemory 202 allows for the provision of pulses of many shapes. Cliniciansare constantly experimenting with finding pulse shapes that will providepatients better therapy, and so such flexibility enables suchexperimentation.

Additionally, timing channel circuitry 200 does not require discretemultiplexers (190, FIG. 2C) to route pulse parameters to the stimulationcircuitry 160, which as noted must handle and drive hundreds of signals,and take up significant layout area and power to drive. Because the datafor each pulse phase is divided into 32-bit words, data bus 204 needonly be 32 bits wide in the illustrated example, which is easier todrive and control.

Another significant benefit to timing channel circuitry 200 is that datastored for each phase can be reduced to eliminate irrelevant values. Asnoted earlier, in the prior art, all phases generally require a completeregister in the register bank 180 (FIG. 2C) (assuming certain dedicatedregisters are not shortened, as discussed earlier), even if a givenphase does not require use of all values in the register, such as theamplitude and electrode values for the interpulse, passive recovery, andquite phases. Dividing the data for each phase into words, and usingcommands to define the number of words for each phase, allows the datafor certain phases to be narrowed from the 96 bits otherwise specifiedfor the registers in the register bank 180. As discussed earlier withrespect to FIG. 5, inactive phases (inter-phase, quite) require thestorage of only one word (32 bits) in the memory 202; passive recoveryonly requires two words (64 bits). The efficiency of reducing the phasedata in this manner becomes more substantial in IPG architectures witheven greater numbers of electrodes. However, it is not strictly toreduce the phase data in this way, and instead every pulse phase couldcomprise a set number of words in the memory 202, even if some of thisdata is not useful for a given phase. In that circumstance, the addressfor the next phase would be known in advance, and it therefore would notbe required to store the next address in word1, as illustrated earlier.

Benefits to the disclosed circuitry are multiplied when timing channelcircuitry 200 is extended to support more than one timing channel, andthe extension to four timing channels is illustrated in FIG. 7. Asshown, only one memory 202 is needed to hold all of the pulse parametersfor the timing channels 150, each of which is denoted by a subscript.Certain circuitry 250 (shown in the dotted lined box in FIG. 4) isrepeated for each timing channel, but other circuitry, such as thecontrol register 208 and stimulation controller 214, can be shared byeach timing channel.

Modifications may be made to timing channel circuitry 200. For example,memory 202 need not be a dual-port RAM supporting two address/databuses: bus 115 that communicates with the control circuitry 110 andbuses 204/224 that communicate with the stimulation circuitry 160.Instead, communications on a single address/data bus could be timemultiplexed. Additionally, the data bus 204 and address bus 224 couldalso comprise a single, time-multiplexed bus.

Memory 202 preferably comprises random access memory, such as static ordynamic random access memory. However, programmable/erasablenon-volatile memory could be used as well, such as Flash memory. Whileprogramming and erasing require additional overhead if the pulseparameters are changed, such therapy changes do not occur frequently,and erasure and re-programming are suitably quick from the patient'sperspective.

Timing channel circuitry 200 can be implemented in the same integratedcircuit upon which the stimulation circuitry 160 is normally fabricated,but can also be discrete therefrom. Although shown in FIG. 4 as discretecircuit hardware blocks, many of the aspects of the circuitry can beimplemented in software or firmware as well, as one skilled in the artwill appreciate.

Timing channel circuitry 200 can be scaled in many different ways.Additional electrodes can be supported beyond the eight illustrated. Forexample, if IPG 100 contains 16 electrodes, additional words can beincluded in the memory 202 for each phase requiring electrode data, suchas the active phases. Or, the width of the memory 202 can be increasedto accommodate longer 64-bit words for example.

Although particular embodiments of the present invention have been shownand described, it should be understood that the above discussion is notintended to limit the present invention to these embodiments. It will beobvious to those skilled in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe present invention. Thus, the present invention is intended to coveralternatives, modifications, and equivalents that may fall within thespirit and scope of the present invention as defined by the claims.

What is claimed is:
 1. An implantable stimulator device, comprising: amemory configured to store pulse parameters for a periodic pulse,wherein each pulse comprises a plurality of sequential pulse phases eachwith a duration; and stimulation circuitry configured to sequentiallyform the pulse phases at electrodes for stimulating a patient's tissue,wherein the memory is addressable via an address bus to sequentiallyprovide via a data bus the pulse parameters to the stimulationcircuitry, wherein the pulse parameters for at least one of the pulsephases is stored in a plurality of addresses in the memory.
 2. Theimplantable stimulator device of claim 1, wherein the pulse parametersfor the plurality of pulse phases do not comprise the same number ofaddresses in the memory.
 3. The implantable stimulator device of claim1, wherein a first address in the memory for the pulse parameters foreach of the plurality of pulse phases comprises a pre-defined datastructure.
 4. The implantable stimulator device of claim 3, wherein thefirst address comprises the duration for each pulse phase.
 5. Theimplantable stimulator device of claim 4, wherein the first addressfurther comprises a command indicative of the number of addresses inwhich pulse parameters are stored in the memory for each pulse phase. 6.The implantable stimulator device of claim 5, wherein the first addressfurther comprises an address in the memory for a subsequent pulse phase.7. The implantable stimulator device of claim 1, wherein the pulseparameters comprise amplitude data and active electrode and electrodepolarity data for at least some of the plurality of pulse phases.
 8. Theimplantable stimulator device of claim 7, wherein the pulse parametersdo not comprise amplitude data and active electrode and electrodepolarity data for at least some of the plurality of pulse phases.
 9. Theimplantable stimulator device of claim 1, wherein the plurality ofaddresses for each pulse phase are sequentially read out of the memoryin accordance with a clock.
 10. The implantable stimulator device ofclaim 9, wherein at least some of the pulse parameters are provided to afirst register, wherein the first register stores information foraddressing the memory and for timing each of the plurality of phases.11. The implantable stimulator device of claim 9, wherein at least someof the pulse parameters are provided to a second register, wherein thesecond register provides the at least some of the pulse parameters tothe stimulation circuitry in parallel.
 12. The implantable stimulatordevice of claim 1, wherein the plurality of pulse phases are selectedfrom the group consisting of an active phase that actively providescurrent at the electrodes, a recovery phase for collecting stored chargebut which does not actively provide current at the electrodes, and aquite phase that does not actively provide current at the electrodes.13. The implantable stimulator device of claim 12, wherein the activephase, the recovery phase, and the quite phase are stored in differentnumbers of addresses in the memory.
 14. An implantable stimulatordevice, comprising: a memory configured to store pulse parameters for aperiodic pulse, wherein each pulse comprises a plurality of sequentialpulse phases each with a duration, wherein the pulse parameters for eachof the plurality of pulse phases are each stored in at least one addressin the memory; a first register for receiving first data from a firstaddress of the at least one address in the memory for each of the pulsephases; a second register for receiving second data from other addressesif present of the at least one address in the memory for each of thepulse phases; control circuitry for receiving the first data, whereinthe first data comprises the duration of each of the plurality ofphases; and stimulation circuitry for receiving the second data, whereinthe second data configures the stimulation circuitry to form the pulsephases at electrodes for stimulating a patient's tissue.
 15. Theimplantable stimulator device of claim 14, wherein the first datafurther comprises information for addressing the memory, wherein theinformation for addressing the memory comprises a command indicative ofthe number of addresses in which pulse parameters are stored in thememory for each pulse phase.
 16. The implantable stimulator device ofclaim 15, wherein the information for addressing the memory furthercomprises an address in the memory for a subsequent pulse phase.
 17. Theimplantable stimulator device of claim 14, wherein the pulse parametersfor each of the plurality of pulse phases do not comprise the samenumber of addresses in the memory.
 18. The implantable stimulator deviceof claim 14, wherein the second data comprises amplitude data and activeelectrode and electrode polarity data for at least some of the pluralityof pulse phases.
 19. The implantable stimulator device of claim 14,wherein the plurality of pulse phases are selected from the groupconsisting of an active phase that actively provides current at theelectrodes, a recovery phase for collecting stored charge but which doesnot actively provide current at the electrodes, and a quite phase thatdoes not actively provide current at the electrodes.
 20. The implantablestimulator device of claim 19, wherein the active phase, the recoveryphase, and the quite phase are stored in different numbers of addressesin the memory.